1. Technical Field
This invention relates to devices and methods of storing and/or reading data on an optical disk, and more particularly to such devices and methods employing techniques wherein the optical disk is encoded using holographic gratings. Further, the present invention relates to such devices and methods wherein more than one holographic grating can be formed in the same portion of the optical disk, thereby increasing the data storage capacity of the disk.
2. Background Art
Optical memories, such as a compact disk-read only memory (CD-ROM), are capable of a relatively large storage density, when compared to other data storage media. These high storage-density memories are revolutionizing the computer industry and have made memory-thirsty multimedia products for publishing and entertainment the hottest-selling computer hardware and software. As an example, a 5-inch CD-ROM disk can store a 30-volume encyclopedia containing not only text and pictures, but also sounds and videos. Optical memories also reduce the space needed for storing exploding data files. The space once needed to store volumes of printed material is no longer necessary. A few, small optical disks take their place. The cost of manufacturing optical disks can also be much less than equivalent paper publishing. For instance, the cost of making a conventional compact disk (CD), in quantity, is less than a dollar. Benefiting from the huge market of audio CDs, CD-ROM technology has become very mature and low cost.
Conventional CD-ROM type optical memories have high storage density, mainly because the space needed to store one bit of digital data corresponds to the size of an optical beam at its point of focus. Of course, the diameter of the optical beam at the point of focus varies with the wavelength of light employed. Typically, this diameter is about 1 .mu.m in current CD-ROM players employing a standard wavelength of about 0.78 .mu.m. Given this diameter, a total of about 650 MB can be stored on a CD (assuming the data is stored in the typically used area between a radius of 59 mm and a radius of 120 mm). This storage capacity is enough for some current audio and computer applications, but it is still too small for high-resolution video applications. To illustrate, the above-described CD can store about 30 minutes of compressed video, which will not hold an average movie. As a result, videos are currently distributed on larger 12-inch disks, which are not compatible with the conventional 5-inch disk CD-ROM players. In addition to video, many other data storage applications are being restricted by the data storage limitations of current CD-ROM systems. Thus, increasing the storage capacity of an optical disk beyond current CD-ROM systems would be advantageous.
One way of further increasing the storage density of an optical memory, such as a CD-ROM, is to use a shorter wavelength, thereby reducing the diameter of the optical beam at its point of focus. Thus, more data bits can be fit onto the optical disk. Much effort has been expended in the development of short wavelength laser diodes, partly for this reason. However, to date no such system has become commercially available.
Another approach to increasing the storage density of an optical memory has involved employing holographic techniques. In a conventional CD-ROM the data bits are represented by pits on a plastic disk coated with a protection layer. The length of the pit encodes the data stored. A laser beam is focused to a very tiny spot (i.e. .about.1.7 .mu.m in diameter) on the optical disk. When the focused beam hits a pit, the reflected intensity is much lower than when only a land is illuminated. An optical head is able to convert the reflections from the pits and lands to appropriate electrical pulses which are then decoded and sent to a buffer memory and finally to the host computer.
Conversely, holographic techniques involve using a hologram to store and reconstruct whole images. This hologram could be an analog image of some type, or a page of digital data. A volume holographic disk is used as the storage medium. In the analog case, the reconstructed image is focused on a detector array or a camera for further processing. In the digital case, the reconstructed bit array would be focused and mapped onto a detector array before processed by decoding circuitry. Reading out an entire page of digital data in this above-described parallel manner would have the advantage of creating a higher data throughput rate. In addition, more data could theoretically be stored per unit area than with conventional CD methods. This latter advantage could be further enhanced by multiplexing techniques which would allow the storage of multiple holograms in the same spot on the disk. However, the aforementioned holographic approach requires sophisticated optics and supporting mechanisms. For example, in the case of the storage of digital data, a precise one-to-one mapping between the elements of the bit array on the disk and the elements of the detector array in the optical head is essential. This requires highly accurate optical heads and drive mechanisms.
In addition, because of the complexity of the systems needed to record and readout a holographic image, e.g. a precisely positioned detector array, existing CD-ROM systems can not be employed. Rather, new, completely unique drives, etc. are required. Thus, the aforementioned holographic approach can not take full advantage of the mature technologies associated with current CD-ROM drives.
More recently, an approach was introduced to increase the storage capacity of conventional CD-ROM system by stacking CDs in pancake fashion. To read from the different layers in this stack CD array, the focus of the readout beam is changed by moving its associated focusing lens(es). The number of CDs that can be stacked in this manner is dependent on the insertion loss of each CD layer. To date, a system with six stacked CDs has been demonstrated. Although stacking increases storage capacity of the overall system, each of the CDs in the stack are still limited to the volume of data described above.
In view of the drawbacks associated with the above-described approaches to increasing the data storage capability of an optical memory, it is desirable that other approaches be developed which can overcome the problems of existing systems.